diff --git a/yaml/_loader_state.ts b/yaml/_loader_state.ts
index d58ecbf385f6..c7574807d89e 100644
--- a/yaml/_loader_state.ts
+++ b/yaml/_loader_state.ts
@@ -451,12 +451,21 @@ export class LoaderState {
 
     for (const [key, value] of Object.entries(source)) {
       if (Object.hasOwn(destination, key)) continue;
-      Object.defineProperty(destination, key, {
-        value,
-        writable: true,
-        enumerable: true,
-        configurable: true,
-      });
+      // `Object.defineProperty` is significantly slower than direct
+      // assignment in V8. Direct assignment produces an identical descriptor
+      // (writable/enumerable/configurable) for ordinary keys; the only
+      // sensitive case is `__proto__`, where direct assignment would mutate
+      // the prototype chain instead of creating an own property.
+      if (key === "__proto__") {
+        Object.defineProperty(destination, key, {
+          value,
+          writable: true,
+          enumerable: true,
+          configurable: true,
+        });
+      } else {
+        destination[key] = value;
+      }
       overridableKeys.add(key);
     }
   }
@@ -523,12 +532,18 @@ export class LoaderState {
         this.#scanner.position = startPos || this.#scanner.position;
         throw this.#createError("Cannot store mapping pair: duplicated key");
       }
-      Object.defineProperty(result, keyNode, {
-        value: valueNode,
-        writable: true,
-        enumerable: true,
-        configurable: true,
-      });
+      // See `mergeMappings` above for why `Object.defineProperty` is kept
+      // only for the `__proto__` key.
+      if (keyNode === "__proto__") {
+        Object.defineProperty(result, keyNode, {
+          value: valueNode,
+          writable: true,
+          enumerable: true,
+          configurable: true,
+        });
+      } else {
+        result[keyNode] = valueNode;
+      }
       overridableKeys.delete(keyNode);
     }
 
diff --git a/yaml/parse_test.ts b/yaml/parse_test.ts
index f875fc4843b6..cc3cd34a477b 100644
--- a/yaml/parse_test.ts
+++ b/yaml/parse_test.ts
@@ -1071,6 +1071,29 @@ Deno.test("parse() throws at reseverd characters '`' and '@'", () => {
   );
 });
 
+Deno.test("parse() does not pollute prototype with `__proto__` key", () => {
+  // A YAML key of `__proto__` must produce an own property on the result,
+  // not mutate the result's prototype chain.
+  const result = parse("__proto__:\n  polluted: true") as Record<
+    string,
+    unknown
+  >;
+  assert(Object.hasOwn(result, "__proto__"));
+  assertEquals(
+    (result as { __proto__: unknown }).__proto__,
+    { polluted: true },
+  );
+  // Same guarantee for the merge type (which goes through `mergeMappings`).
+  const merged = parse(`<<:
+  __proto__:
+    polluted: true
+ok: 1`) as Record<string, unknown>;
+  assert(Object.hasOwn(merged, "__proto__"));
+  assertEquals(merged.ok, 1);
+  // Sanity: an unrelated object's prototype is untouched.
+  assertEquals(({} as { polluted?: unknown }).polluted, undefined);
+});
+
 Deno.test("parse() handles sequence", () => {
   assertEquals(parse("[]"), []);
   assertEquals(